1) Field of the Invention
The present invention relates to a semiconductor laser module and a Raman amplifier that has reduced relative intensity noise (RIN).
2) Description of the Related Art
Following the recent rapid spread of the Internet and the rapid increase of in-company LAN connection, an increase in data traffic appears as a problem. A dense-wavelength division multiplexing (DWDM) transmission system which has been developed to prevent a decrease in communication performance due to increased data traffic is spreading fast.
The DWDM transmission system realizes a large capacity transmission that transfers 100 times larger signal than the conventional system by carrying multiple optical signals on different wavelengths on a single fiber. The existing DWDM transmission system particularly enables wideband, long-distance transmission using an erbium-doped fiber amplifier (EDFA). The EDFA, Er-doped optical fiber, amplifies signals with the wavelength of 1550 nm when transmits the laser with the wavelength of 1480 nm or 980 nm.
On the other hand, The EDFA is a centralization type optical amplifier which centralized the excitation of an optical signal. The EDFA has, therefore, restrictions which include noise accumulation due to the loss of a transmission optical fiber, and signal distortion and noise generation due to the non-linearity of a transmission optical fiber. In addition, since the EDFA enables amplifying light in a wavelength band only set by the Er band gap energy, it is difficult to widen band to realize further multiplexing.
In these circumstances, attention is paid to a Raman amplifier as an optical fiber amplifier to replace the EDFA. The Raman amplifier is a distribution type optical amplifier which does not require a special fiber such as the erbium-doped fiber as required by the EDFA but employs an ordinary transmission fiber as a gain medium. Therefore, the Raman amplifier realizes uniform gain in a wider transmission band than that of the conventional EDFA-based DWDM transmission system.
FIG. 18 is a block diagram which shows the configuration of a conventional Raman amplifier employed by the DWDM transmission system. Each of semiconductor laser modules, 183a to 183d, includes a Fabry-Perot type semiconductor laser device. Each of semiconductor laser modules 183a to 183d emits a laser beam, from which an excitation light is generated, to respective polarization synthesis couplers 61a and 61b. The laser beams emitted by the semiconductor laser modules 183a and 183b have same wavelength, however, the polarization synthesis coupler 61a makes their polarization surfaces different from each other by 90°. Likewise, the laser beams emitted by the semiconductor laser modules 183c and 183d have same wavelength, however, the polarization synthesis coupler 61b makes their polarization surfaces different from each other by 90°. The polarization synthesis couplers 61a and 61b transmit the polarization synthesized laser beams to a WDM coupler 62, respectively. It is noted that the laser beams emitted from the polarization synthetic couplers 61a and 61b had different wavelengths.
The WDM coupler 62 synthesizes the laser beams from the polarization synthesis couplers 61a and 61b. A laser beam from the WDM coupler 62 passes through an isolator 60 and a WDM coupler 65, and then is incident on an amplification fiber 64 as an excitation light. While the amplification fiber 64 amplifies target optical signal which is input from a signal light input fiber 69 through an isolator 63 to pass, the amplification fiber 64 combines the signal light with the excitation light and Raman-amplifies the combined signal.
Raman-amplified optical signal in the amplification fiber 64 is transmitted into a monitor light distribution coupler 67 trough the WDM coupler 65 and an isolator 66. The monitor light distribution coupler 67 emits a part of the amplified optical signal to a control circuit 68 and the remainings to an optical signal output fiber 70 as output light
The control circuit 68 controls the light emitting states, e.g., light intensities, of semiconductor laser devices 180a to 180d based on the partially input the amplified optical signal into itself, and the control circuit 68 controls feedback whether the gain band of the Raman amplification is flat. FIG. 19 is a longitudinal sectional view which shows the configuration of the semiconductor laser module employed in the conventional Raman amplifier. In FIG. 19, a semiconductor laser module 183 includes a Peltier module 200 which is arranged on the inner bottom of a package 202 formed out of Cu—W alloy or the like. A base 197 is placed on the Peltier module 200, a carrier 198 is on the base 197, and a sub-mount 199 is on the carrier 198. Further, a semiconductor laser device 180 is positioned on this sub-mount 199.
Electric current is applied to the Peltier module 200 to thereby heat or cool the semiconductor laser device 180 according to the polarity of the applied current. However, the Peltier module 200 is generally used as a cooler to prevent an oscillation wavelength variance caused by the temperature increase of the semiconductor laser device 180. In other word, if a laser beam has a longer wavelength than a desired wavelength, the Peltier module 200 heats up the semiconductor laser device 180. Or, if a laser beam has a shorter wavelength than the desired wavelength, the Peltier module 200 cools down the semiconductor laser device 180. Specifically, this temperature control is based on a value detected by a thermistor (the figure is omitted), which is located on the sub-mount 199 and near the semiconductor laser device 180. A controller (also the figure is omitted) controls the Peltier module 200 so as to keep the temperature of the semiconductor laser device 180 constant.
On the base 197, not only the carrier 198 but also a first lens 192, an isolator 193 and a monitor photodiode 196 are placed. The laser beam emitted from the semiconductor laser device 180 is converged by a second lens 194 through the first lens 192 and the isolator 193. The laser beam converged by the second lens 194 is introduced into an optical fiber 203 which is fixed by a ferrule 201. The monitor photodiode 196 monitors and detects light leaked from the reflection coating of the semiconductor laser device 180.
Another example of the conventional Raman amplifier is explained. FIG. 20 is a block diagram which shows the configuration of another example of the conventional Raman amplifier which is employed in the DWDM transmission system. In FIG. 20, the same constituent devices as those in FIG. 18 are denoted by the same reference numerals and are not be described herein. The Raman amplifier shown in FIG. 20 differs from the Raman amplifier shown in FIG. 18 only in that the semiconductor laser modules 183a to 183d are replaced by semiconductor laser modules 182a to 182d, respectively. The semiconductor laser module 182a consists of a Fabry-Perot type semiconductor laser device 180a and a fiber grating 181a. Likewise, the other semiconductor laser modules 182b to 182d consist of Fabry-Perot type semiconductor laser devices 180b to 180d and fiber gratings 181b to 181d, respectively.
FIG. 21 is a longitudinal sectional view which shows the configuration of the semiconductor laser module employed in the other example of the conventional Raman amplifier described above. In FIG. 21, the same constituent devices as those in FIG. 19 are denoted by the same reference numerals, and are not described herein. The semiconductor laser module shown in FIG. 21 includes a fiber grating 181 at a predetermined position of an optical fiber 203. What differs from the semiconductor laser module in FIG. 19 is that a laser beam converged by a second lens 194 is introduced into the optical fiber 203 fixed by a ferrule 201 and then introduced to the fiber grating 181.
FIG. 22 is an explanatory view which explains the structure of the semiconductor laser device and the function of the fiber grating in the semiconductor laser module shown in FIG. 21. In FIG. 22, the semiconductor laser device 180 has an active layer 221. The active layer 221 has an optical reflection surface 222 on one end and an optical emission surface 223 on the other end. The light generated in the active layer 221 is reflected by the optical reflection surface 222 and emitted from the optical emission surface 223.
As shown in FIG. 21, the optical fiber 203 is arranged to face the optical emission surface 223 of the semiconductor laser device 180. The optical fiber 203 is optically coupled with the optical emission surface 223 of the semiconductor laser device 180. The fiber grating 181 located at a predetermined position relative to the optical emission surface 223 is formed in a core 232 in the optical fiber 203. The fiber grating 181 selectively reflects the laser beam with a specific wavelength. Namely, the fiber grating 181 functions as an external resonator, and a resonator is formed between the fiber grating 181 and the optical reflection surface 222. The laser beam that is selected by the fiber grating 181 is amplified and emitted as an output laser beam 241.
However, in each of the semiconductor laser module 183 shown in FIG. 19 and the semiconductor laser module 182 shown in FIG. 21, the laser beam emitted from the semiconductor laser device 180 may possibly be reflected by the incident surface of the first lens 192, by the incident surface of the isolator 193 by and the incident surface of the second lens 194. The reflected beam is incident on the semiconductor laser device 180 as return beam, which causes an increase of RIN. Further, as shown in FIGS. 19 and 21, since the incident end surface of the optical fiber 203 is perpendicular to the optical axis of the incident laser beam, the reflected beam on the end surface is also a factor for RIN increase.
Amplification occurs fast particularly in Raman amplification. Therefore, if the intensity of excitation light fluctuates, Raman gain also fluctuates. This Raman gain fluctuation results in fluctuation in the intensity of an amplified signal, which disadvantageously hampers Raman amplification.
In the semiconductor laser module 182 shown in FIG. 21, since the distance between the fiber grating 181 and the semiconductor laser device 180 is large, the RIN is increased by the resonance between the fiber grating 181 and the optical reflection surface 222. This is because, in an RIN spectrum, peaks occur at the every frequency which corresponds to the reciprocal of the time what takes for the beam to round-trip between the optical reflection surface 222 of the semiconductor laser device 180 and the fiber grating 181. Because of the fast amplification in Raman amplification, if the intensity of the excitation light fluctuates, Raman gain also fluctuates. This Raman gain fluctuation results in the fluctuation of the amplified signal intensity, which disadvantageously hampers stable Raman amplification.
In the semiconductor laser module 182 shown in FIG. 21, it is necessary to optically couple the optical fiber 203 which includes the fiber grating 181 with the semiconductor laser device 180. To do so, it takes time and labor for optical axis alignment during assembly. In addition, since this optical coupling is mechanical optical coupling in the resonator, the oscillation characteristic of the laser may possibly change according to mechanical vibration, which may disadvantageously make it impossible to provide stable excitation beam.
FIG. 23 shows the RIN characteristic of the semiconductor laser module shown in FIG. 21. FIG. 24 shows a measurement system with which the RIN characteristic shown in FIG. 23 is obtained. As shown in FIG. 24, a semiconductor laser module 300 is driven by a laser module driver 302. Laser beam emitted from the module 300 is attenuated by an optical attenuator 303 and falls on an optical signal analyzer 304. This optical signal analyzer 304 measures the RIN. Products HP70810B and HP70908A (Hewlett-Packard Co.) are particularly used here as the optical signal analyzers 304. The optical signal analyzers 304 adjust the optical attenuator 303 so as to be able to input light of 2.5 dBm while an internal attenuator of each analyzer set at 0 dB. More detailed measurement conditions are as follow. The resolution band width is automatically set in a measurement range of 0 to 22 GHz, set at 0.3 MHz in a measurement range of 0 to 2 GHz, set at 0.3 MHz in a measurement range of 0 to 0.1 GHz, and sets at 0.464 MHz in a measurement range of 0 to 0.01 GHz. As shown in FIG. 23, the result of the RIN measurement under the measurement conditions shows that the RIN characteristic is deteriorated with a driving current Iop is equal to 900 mA.
Meanwhile, there are a forward excitation type Raman amplifier which excites optical signal forward and a bidirectional excitation type Raman amplifier which excites signal light from two directions as well as the forward excitation type Raman amplifier, shown in FIGS. 18 and 20, which amplifies optical signal backward. Recently, the backward excitation type Raman amplifier is used more frequently than the other types. The reason is as follows. The forward excitation type Raman amplifier in which weak optical signal travels in the same direction along with the strong excitation light has disadvantages that a fluctuation in the intensity of the excitation light tends to move to the optical signal, such a nonlinear effect as four-optical-wave mixture tends to occur, and the polarization dependency of the excitation light tends to appear. Therefore, the intensity of an excitation optical source (which consists of the semiconductor laser module, the polarization synthetic coupler and the WDM coupler) which is employed in the forward excitation type Raman amplifier cannot increase. Compared to the intensity of the excitation optical source which is employed in the backward excitation type Raman amplifier, the forward excitation type Raman amplifier is required to be actuated at a low excitation light intensity. However, if the driving current of the semiconductor laser device 180 goes down too low in order to reduce the excitation optical intensity, it appears that the influence of relaxation oscillation on the low frequency range of the RIN, therefore the RIN increases. As a result, it is desired to stabilize an excitation optical source in the forward excitation type Raman Amplifier. Further, the Raman amplifier shown in FIG. 20 has a disadvantage in that the presence of the fiber grating limits applicable excitation types.
Raman amplification is under the condition that the direction of the polarized optical wave is on the same direction of the excitation light. That is, since the gain of the Raman amplification has polarization dependency, it is necessary to decrease the effect of a variance between the directions of polarized optical wave and the excitation light. In the backward excitation type Raman amplifier, the polarization of the optical wave is at random during propagation, so it causes no problem.
However, in the forward excitation type Raman amplifier, the gain has a high polarization dependency which, therefore, needs to be reduced by orthogonal synthesis, depolarization or the like of the excitation light. In other words, it is necessary to decrease degree of polarization (DOP).